Corrosion is an inevitable problem in most water containing and operating systems such as steam producing boilers. This detrimental phenomenon is particularly destructive in steam generating nuclear fission reactors which present an environment of radiation as well as high temperatures accentuating the deleterious effects of the water upon many metal components.
Moreover, corrosion can constitute an exceedingly complex problem as to its source and effects upon structural materials, and the particular environment. One rather distinctive type of corrosion which has been found to occur in the stainless steel piping and other water containing vessels in nuclear reactor plants has been designated intergranular stress corrosion cracking. This type of corrosion is considered to be attributable to the stainless steel metal having become sensitized by high heat, such as from welding joints, and subsequently subjected to both mechanical stress and a corrosive environment, as well as the high temperatures and radiation encountered within and about a steam generating water cooled nuclear fission reactor plant.
The occurrence of such stress corrosion cracking has been found to be more prevalent or aggressive in higher oxidizing environments. High oxygen concentrations in nuclear fission reactor water coolant is a common condition due to the irradiation induced decomposition of some water into its components of oxygen and hydrogen. To counter the corrosive effects of a high oxidizing environment attributable to such radiation disassociation of water, it has been proposed to add hydrogen to reactor water coolant which will reduce free oxygen in the water by combining with it and thereby minimize its corrosive inducing effects. For example, under typical water chemistry conditions, the oxygen concentration is approximately 200 parts per billion and the hydrogen concentration is approximately 10 parts per billion. The concentration of oxygen and hydrogen found to be required for effective prevention of intergranular stress corrosion cracking is in the approximate range of about 2 to 15 parts per billion of oxygen and about 100 parts per billion hydrogen.
Corrosion control through manipulation of the free hydrogen and/or oxygen content of reactor water coolant is an established procedure.
Although an effective measure for controlling corrosion within the water system of a nuclear fission reactor, the addition of hydrogen to reactor water coolant for the purpose of suppressing the free oxygen content also fosters the conversion of nitrate and nitrite compounds within the water coolant to ammonia. This hydrogen promoted conversion of nitrogen containing compounds to volatile ammonia presents an ancillary problem within the radioactive environment of a nuclear fission reactor due to the radiation induced transmutation of oxygen, by the O.sup.16 (n,p) reaction, into the nitrogen-16 isotope. Although nitrogen-16 is a radioactive nuclide with a half-life of only approximately 7 seconds, about 6 MeV gamma ray is emitted therefrom in its decay. The level of intensity and the relatively high energy of such gamma radiation would require significant shielding to protect personnel from the radiation field. Thus, when this gamma ray emitting nitrogen isotope derived from oxygen is incorporated in a compound which is then converted into volatile ammonia, it becomes a significant source of radiation which can be transported along with steam throughout a steam or vapor system.
Boiling water type of nuclear fission reactors, unlike pressure water reactors, produce steam initially within the reactor pressure vessel from the reactor coolant water surrounding at least a major portion of the heat producing core of fissionable nuclear fuel. This steam is conveyed directly from the fissioning fuel core containing reactor pressure vessel to its designated location of work such as a steam turbine for electrical power generation. Thus, differing from the pressure water type reactor where hot pressurized water from the reactor pressure vessel passes through a heat exchanger which in turn produces steam, the steam from a boiling water reactor passes directly to and through the turbine system and generating facility before returning by way of the circuit to the nuclear reactor pressure vessel for repeating the cycle.
Accordingly, when treating the coolant water in a boiling water nuclear fission reactor by the addition of hydrogen to control corrosion, there occurs a combination of conditions which may raise the radiation level in a nuclear plant facilities at locations beyond the reactor containment structure, namely, within the steam turbine electrical generating unit. For instance, it appears that an increase in hydrogen concentration of the coolant water will foster oxidation of nitrogen containing compounds in the coolant water to volatile ammonia, including those comprising radioactive nitrogen-16 isotope produced from oxygen. The ammonia, containing gamma ray emitting nitrogen isotopes, being readily volatile, will be carried along with the steam out from the reactor pressure vessel into and through steam conduits and the turbines of the generator, where it decays in the turbine condenser system.
Such a potentially adverse condition can significantly increase the reactors construction and operating costs because of a need for added radiation shielding and more stringent limitations on personnel exposure time in carrying out normal facilities operations and maintenance.